277 volt power at home

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
Post Reply
User avatar
Rich Feldman
Posts: 944
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

277 volt power at home

Post by Rich Feldman » Sat Jun 09, 2018 9:20 pm

Just took another step toward what might become a real fusor power supply. Have long wanted 277 volt power for some nice new 60 mA NST's, bought cheaply on ebay 'cause of their primary voltage. Then last month, the 277 volt parking lot lights at work were changed from sodium to LED. Facility manager had the old luminaires stacked by the dumpsters. Pile gradually dwindled to nothing, but not before I took a couple home to play with. (Each unit is about 12 x 40 inches, with a 32 inch long lamp.)

Today's story might sound familiar on the weldingweb forum. My house has two 240V receptacles, one in the kitchen and one in garage for clothes dryer. The dryer has a now-deprecated 3 wire cord with NEMA 10-30 plug (hot-hot-neutral, no separate grounding conductor). But the wire run is entirely enclosed in steel conduits and boxes.

I spliced a spare dryer cord onto a long 10-AWG 4-conductor cord. The fourth wire is extended along the dryer cord and terminated with a ring lug, for a removable bond to the electrical box.
DSCN0544.JPG
Time to verify that the steel enclosure could safely serve as an "intentional low resistance groundING conductor", with no local connection to the groundED conductor (white wire). Today's code doesn't permit my new copper water pipes to be used for grounding. Any wire connected to a water pipe is there to divert electricity out of the pipe, to protect plumbing users. Not to divert electricity into the pipe, to protect electricity users. And ground rod connections aren't sufficient -- in the event of a hot to ground short, the earth resistance is too high to keep voltage at a safe level, and to draw enough current to immediately trip the overcurrent protection device.

This called for a milliohmeter -- the opposite of a megger. With the dryer circuit turned off, the groundED and groundING wires at end of new cord were connected to a low-voltage, high-current transformer controlled by a variac.
DSCN0547.JPG
It took 2.7 volts to get 15 amps. Outside of the picture, circuit runs through the neutral wire back to ground bar at the breaker panel, then returns through the steel conduit and cord wire #4.

Voltages were measured with respect to breaker panel ground, accessible via the neutral of a nearby 120V circuit that was turned off.
Then we could compute the resistances, including the effects of any temperature rise from carrying 15 A.
Flexible cord dropped 320 mV in each wire (21 milliohms).
Steel pipe dropped 122 mV (8 milliohms).
Neutral wire inside the pipe dropped 1700 mV (122 milliohms).

Neutral contact from plug blade to fixed wire dropped 219 mV (14 milliohms).

The neutral wire resistance was much higher than expected. Then I found that it's only 12 AWG, unlike the black and red wires.
Still higher than expected. I found that the clip-on ammeter was indicating 15 A when actual current was 18 A.
Voltage drops posted above are based on 15.2 A measured with a current shunt resistor.

Next step: connect the big variac or a boost transformer, get 277 V, light up some sodium vapor.
Last edited by Rich Feldman on Sun Jun 10, 2018 3:42 am, edited 1 time in total.
Richard Feldman

User avatar
Rich Feldman
Posts: 944
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: 277 volt power at home

Post by Rich Feldman » Sun Jun 10, 2018 3:10 am

Yay! First suitable variac in the shed was not the 240 volt unit with motorized adjuster, it's a dual 120 that I'd forgotten about, and never used.
Serious Tesla coilers like RH probably know the type. Nameplate of lower unit says STACO ENERGY PRODUCTS CO. / DAYTON, OH. U.S.A. / INPUT: 240 V 50/60 Hz / OUTPUT: 0-280 V 25 A 7.0 KVA. One nice thing about the dual, is it keeps the midpoint of output voltage at ground potential.

Initial connection includes a 10 amp two pole circuit breaker / power switch. I was afraid it might trip from inrush current when variac was switched on, but no prob so far. Power-on indicator is a gratifying hum. With no load, the knob goes up to 290 V. No big wow -- clock radios in the UK have 240 volts in the cord. I adjusted it to 277 V.
DSCN0550.JPG
Then lost little time connecting a parking lot luminaire, maintaining its designed orientation. It turned on uneventfully, starting with a deep red glow (neon fill gas?) and warming up to familiar yellow over a few minutes. Less than 1 amp from wallplug, if clip on meter is to be trusted on its 0-6 range. Parking lot used to have two lights on each of more than a dozen poles.
DSCN0548.JPG
DSCN0554.JPG
One next step is to improve the wiring enclosure. Covers, grommets & strain reliefs, crimped-on wire terminals, panel meters! Shopping list includes a new crimping tool -- apparently you really can squeeze too hard by hand, or I used the wrong slot for yellow insulated barrels on 10 AWG wire.
DSCN0552.JPG
A simple boost transformer would be much quieter and more portable than the biggish variac. Or just feed the lamp units or NST's with straight 240 (which runs a bit high here). The lamp ballasts have two tap options: 277 and 208.
Richard Feldman

User avatar
Bob Reite
Posts: 275
Joined: Mon Aug 26, 2013 1:03 am
Real name: Bob Reite
Location: Wilkes Barre/Scranton area

Re: 277 volt power at home

Post by Bob Reite » Sun Jun 10, 2018 3:59 am

Set the lamp to 208 and run it at reduced voltage using a buck transformer. Too bad it doesn't have a tap for 230 or 240 volt
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

User avatar
Rich Feldman
Posts: 944
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: 277 volt power at home

Post by Rich Feldman » Sun Jun 10, 2018 7:24 pm

Right-o, Bob! Small buck or boost transformer stories to follow, but not before the real-world exercises.

Here's the ballast I got Friday night, from the very last luminaire. Its 43" long lamp was broken. In its place is a 29" "stubby" lamp previously salvaged from a different unit. Yellow glow hurts the eyes to look at, even in bright sunlight.
DSCN0561.JPG
The U-shaped arc tube has little dimples in which sodium can condense. There's always condensed metal in there, even if molten -- I think Na is one of those metals with exceptionally low vapor pressure at its melting point. Along with Hg, whose vapor is also handy in electrical devices. Except of course when the tube breaks and fills with air!
DSCN0566.JPG
Near the U-tube cathode ends, there's a progression of metal amounts in the dimples.
DSCN0567.JPG
Richard Feldman

User avatar
Rich Feldman
Posts: 944
Joined: Mon Dec 21, 2009 11:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: 277 volt power at home

Post by Rich Feldman » Thu Jun 14, 2018 6:12 am

Hey Bob! An ad-hoc boost transformer, cascaded with an ad-hoc autotransformer,
has produced a tolerable example of 277 VAC at home with no variac, no 240 volt plug, and no semiconductors.

It being a problem that only a transformer nut would enjoy, it sure helps to have a transformer collection in the shed. This magnificent specimen has served from time to time as an isolation transformer (not 1:1, or even close) for experiments with saturation and fluxmetering in other transformers. The colored wire harness is left from whatever equipment it was pulled from. The thin green wire is an old test winding with a known turns count.
DSCN0595.JPG
Initial measurement, with 120 V applied between terminals 1 and 2: Terminals 3-4-5 are center-tapped 165 V, and 6-7-8 are center-tapped something between 50 and 70 V.
DSCN0594.JPG
Couldn't have asked for a better ratio. Coloring outside the lines, terminals 3 and 5 are connected to the 120 V supply. 5 is also tied to 1. The 1-2 secondary voltage is almost perfect for boosting 120 V to 208 V, measured between 3 (common) and 2. No new soldering! :-)

That's stage 1. Good enough for the sodium light units, if they were opened up & modified to use the 208 V pigtail wire instead of the 277 V wire.

But I had one extra coil-and-core unit from the luminaire graveyard. It was found there loose, after someone else had dropped it or dropped something on it. Secondary coil had a deep gouge, exposing severed copper windings. Not hard to disconnect and make sure it had no shorted turns.
Then the primary's Common, 208 V, and 277 V pigtails served as an autotransformer. Voltages measured with no load: Stage 1 input 120.9 V, interstage connection 207.9 V, stage 2 output 277.2 V. They didn't change much when the ballast secondary coil (the surviving 90% on long side of known break) was shorted.
DSCN0598.JPG
It looks super clean in the picture, after being taken apart. Four welds cut with hacksaw, producing five blocks of laminated steel. Both coils were removed from the center block without any new damage to the windings. Doing that once in a lifetime is enough, and I wish I'd done it at 31 instead of 62. Details and application to follow.
Richard Feldman

Post Reply